Improve integration with installLiberty task

[jgawor]

Improve integration with the installLiberty task so that wlpDir does not need to be set. For example, want to do something like:

liberty {
    serverName = 'jaxrsSample'
    userDir = '../async-jaxrs-wlpcfg'
    install {
        type = "javaee7"       
    }
}

And as long as installLiberty task is called, rest of the tasks such as libertyStart, etc. should work without any additional configuration.

[jgawor]

Done.